If you're running a system where the local systat does not offer PF related views and you are interested in keeping an eye on what passes in to and out of your network, Can Erkin Acar's pftop is a very useful tool. The name is a strong hint at what it does - pftop shows a running snapshot of your traffic in a format which is strongly inspired by top(1):
pfTop: Up State 1-21/67, View: default, Order: none, Cache: 10000 19:52:28 PR DIR SRC DEST STATE AGE EXP PKTS BYTES tcp Out 194.54.103.89:3847 216.193.211.2:25 9:9 28 67 29 3608 tcp In 207.182.140.5:44870 127.0.0.1:8025 4:4 15 86400 30 1594 tcp In 207.182.140.5:36469 127.0.0.1:8025 10:10 418 75 810 44675 tcp In 194.54.107.19:51593 194.54.103.65:22 4:4 146 86395 158 37326 tcp In 194.54.107.19:64926 194.54.103.65:22 4:4 193 86243 131 21186 tcp In 194.54.103.76:3010 64.136.25.171:80 9:9 154 59 11 1570 tcp In 194.54.103.76:3013 64.136.25.171:80 4:4 4 86397 6 1370 tcp In 194.54.103.66:3847 216.193.211.2:25 9:9 28 67 29 3608 tcp Out 194.54.103.76:3009 64.136.25.171:80 9:9 214 0 9 1490 tcp Out 194.54.103.76:3010 64.136.25.171:80 4:4 64 86337 7 1410 udp Out 194.54.107.18:41423 194.54.96.9:53 2:1 36 0 2 235 udp In 194.54.107.19:58732 194.54.103.66:53 1:2 36 0 2 219 udp In 194.54.107.19:54402 194.54.103.66:53 1:2 36 0 2 255 udp In 194.54.107.19:54681 194.54.103.66:53 1:2 36 0 2 271
Your connections can be shown sorted by a number of different criteria, among others by PF rule, volume, age and so on.
This program is not in the base system itself, probably because it is possible to extract equivalent information using various pfctl options. pftop is however available as a package, in ports on OpenBSD and FreeBSD both as sysutils/pftop, on NetBSD via pkgsrc as sysutils/pftop.